1

Not necessarily. It depends what bus you are getting from Mendoza. But the two main bus stations are are in adjoining blocks, Terminal Alameda for Pullman and Turbus and Terminal Santiago for other companies. Turbus actually has a hotel above the station which isn't bad and Ibis has opened a hotel right next to the other station. So either would do.
http://www.ibis.com/gb/hotel-7531-ibis-santiago-estacion-central/index.shtml
http://www.turbus.cl/turbus/opencms/04_Turismo/0402_tur_hotel_express/santiago.html

It isn't a great neighbourhood but the Universidad de Santiago metro station, line 1, is right there. You could stay anywhere on line 1 if you wanted. You just have to avoid rush hour. The buses leave every 15 minutes or so from either station.

2

I know the budget Residencial Mery, it's on Republica, about 20 minutes walk (or by metro) from the bus terminals.

3

Thank you.........

4

You could always get the bus straight on to Valparaiso, they go regularly and though you'll have done a long journey, it's not too much further to lovely Valparaiso.

5

Just there in March 2013. Easiest way we found was to taxi to the Transvip terminal and take their bus to Valpo. Leaves 3-4 times an hour. Takes about 90 minutes. Then take a taxi to your hotel in Valpo.

6

Transvip just does transfers from the airport to any address of your choosing. I think you mean the Turbus terminal, Terminal Alameda. Across the side street is Terminal Santiago, with other companies which do frequent buses to Valpo. If you are getting any bus except Turbus from Mendoza (Andesmar, Cata), that is the terminal where you are likely to end the trip. Or, as mentioned, just get a Mendoza to Valparaiso bus and bypass Santiago.